The present invention is directed, in general, to a method of using a catalytic cracker and, more specifically, to a method of producing synthesis gas from a regeneration of spent cracking catalyst.
Catalytic cracking processes have been developed principally for upgrading feedstock derived from natural or synthetic crude oil to more valuable hydrocarbon mixtures, particularly of lower molecular weight. These lower molecular weight hydrocarbons are generally more desirable because of their higher quality and market value. In a typical catalytic cracking process, a crude oil derived feedstock is contacted with a hot, regenerated catalyst, at temperatures ranging from about 1200xc2x0 F. to about 1400xc2x0 F. and low to moderate pressures. The chemical reactions that take place in the presence of the catalyst include predominantly scission of carbon-to-carbon bonds (simply cracking), isomerization, polymerization, dehydrogenation, hydrogen transfer, and others, generally leading to lower molecular weight hydrocarbon products.
Some of the cracking reactions in the catalytic cracker also produce hydrocarbonaceous compounds of high molecular weight, of very low volatility, of very high carbon content and of low combined hydrogen content. The hydrocarbonaceous compounds tend to be deposited on the active surfaces of the cracking catalyst and mask the active sites, rendering the catalyst less active, thus unsuitable for continued cracking without regeneration. Deposits of the hydrocarbonaceous matter and the inclusion of absorbed and adsorbed hydrocarbons, as well as the vaporous combustible components in the fluidizing media between the solid catalyst particles, collectively called xe2x80x9ccoke,xe2x80x9d are in a sense undesirable. In response to the undesirable buildup of coke on the surfaces of the catalyst, the oil and gas industry has developed several techniques to reduce, or remove, such buildups.
One technique currently used to reduce the coke forming characteristics of feedstocks, includes without limitation, hydrotreatment, distillation, or extraction of the natural or synthetic crude feedstock prior to charging it to the catalytic cracker. Hydrotreatment, distillation, or extraction of the crude oil derived feedstock serves to remove a substantial amount of the coke precursors, such as contained in asphaltenes, polynuclear aromatics, etc., prior to catalytic cracking. Hydrotreatment, distillation, or extraction are somewhat effective in reducing or removing large amounts of coke precursors from the crude oil derived feedstock, however, such processes are expensive and time-consuming processes. Currently, incrementally available crude oil is of high residuum content and of higher coke forming characteristics at a time when it is unpopular or unlawful to utilize this additional residuum as fuel oil. At the same time, the market for residuum products, other than as fuel oil, is saturated. Additionally, to upgrade the residuum materials by the available technology results in products of lower quality (and lower market value) than would be achieved by catalytic cracking, provided the coke yield can be handled. Moreover, current and anticipated Federal and State Legislation has, and is, scrutinizing the environmental and storage issues associated with use, removal or conversion of the coke precursors. Therefore, there is a great need for an environmentally responsible conversion of the residuum portion of crude oil.
Another technique currently used to remove coke formation from the spent cracking catalyst is to burn the coke away from the catalyst surface using an oxygen-containing gas stream in a separate regeneration reactor. In such a situation, air, oxygen, carbon dioxide, and steam for diluent as combustion gas, may be introduced into the spent cracking catalyst in the lower portion of the regeneration zone(s), while cyclones are provided in the upper portion of the regeneration zone for separating the combustion gas from the entrained catalyst particles. The coke buildup removal process attempts to substantially remove the coke buildup, and is generally effective, but large amounts of greenhouse gases are produced, at least some of which are released into the atmosphere, which is generally environmentally undesirable. Another technique teaches the use of a waste heat boiler as a means of reducing greenhouse gasses from going to the atmosphere, however, the reduction by this method remains limited to the achievable concentration of a fired heater. U.S. Pat. No. 4,388,218 entitled xe2x80x9cRegeneration of Cracking Catalyst in Two Successive Zonesxe2x80x9d to Rowe, and U.S. Pat. No. 4,331,533 entitled xe2x80x9cMethod and Apparatus for Cracking Residual Oilsxe2x80x9d to Dean et al., further detail such processes and are included herein by reference.
Similarly, the regeneration zone must be carried out in such a way that it is in thermal equilibrium with the cracking reaction zone. In other words, the sensible heat of the hot regenerated catalyst in the catalytic cracker should be in balance with the heat requirements of the catalytic cracking reactor zone. In conventional operations, excluding the use of internal or external cooling coils for removing heat from the regenerator reaction zone, coke yield of only about 5 to about 8 weight percent of the total feed may be burned from the catalyst, without exceeding the amount of heat required to balance and sustain the cracking reaction.
Thus, to maintain the thermal balance needed to operate the catalytic cracker and remove enough of the coke from the catalyst to sustain the cracking process, one of two things should be done. First, the amount of coke that forms on the surface of the catalyst should be reduced. However, as mentioned above, this can typically be accomplished by using higher quality feedstock, which is more costly, or subjecting the currently available feedstock to the previously mentioned upgrading, such as but not limited to, hydrotreatment, distillation or extraction processes, which are also more costly. Second, internal or external cooling units could be installed in the regeneration units. However, such internal or external cooling units are costly and unreliable.
Accordingly, what is needed in the art is a method of catalytically cracking crude oil derived feedstock having high coke forming characteristics, without experiencing the drawbacks of the prior art methods.
To address the above-discussed problems of the prior art, the present invention provides a method of producing a synthesis gas from a regeneration of a spent cracking catalyst. The method includes introducing a spent cracking catalyst into a first regeneration zone in a presence of a first oxygen and carbon dioxide atmosphere and at a first regeneration temperature. For example, a temperature that does not exceed about 1400xc2x0 F., and more preferable, a temperature that ranges from about 1150xc2x0 F. to about 1400xc2x0 F., may be used as the first regeneration temperature. The method further includes introducing the spent cracking catalyst from the first regeneration zone into a second regeneration zone. The spent cracking catalyst is introduced into the second regeneration zone in a presence of a second oxygen and carbon dioxide atmosphere, and at a second regeneration temperature substantially greater than the first regeneration temperature. A synthesis gas may then be formed from oxidation of the carbon on the coke located on the spent cracking catalyst within the second regeneration zone. The method further includes introducing the spent cracking catalyst from the second regeneration zone into a third regeneration zone in a presence of a third oxygen and carbon dioxide atmosphere, wherein the third regeneration zone is operated at a temperature of greater than about 1400xc2x0 F. and maintained in an oxidation mode to restore the full potential of the cracking catalyst activity prior to its reuse in the cracking reaction zone, and to produce enough carbon dioxide to support the first, second or third, or any combination thereof, oxygen and carbon dioxide atmospheres.
In contrast to the prior art catalytic cracking method, the above-mentioned method is capable of producing commercial amounts of synthesis gas, which may then be commercially used or sold. Moreover, the above-mentioned method is capable of accepting feedstock having high coke forming characteristics, which in one advantageous embodiment, may be accepted without hydrotreating, separating as a distillation overhead product, or solvent extracting as an extract product the feedstock prior to catalytic cracking. Both the ability to accept, and the ability to accept without the need for hydrotreating, distillation, or solvent extraction, provide both economical and environmental benefits not achieved in the prior art methods.
In another aspect of the invention, the synthesis gas comprises carbon monoxide. Where the amount of synthesis gas produced is inadequate to meet the market needs or the intended system consumption capacity, in an alternative aspect, a supplemental fuel, such as a hydrocarbonaceous material, may be introduced into the spent catalyst flow path through the regeneration zones. Preferably this is added at or near the entrance to the first regeneration zone, in order that the combined hydrogen content will be reduced in the first regeneration zone, thus subsequent regeneration zones will have reduced water formation therein. The supplemental fuel is preferably one of low hydrogen content and is preferably introduced into the first regeneration zone. At least a portion of the first oxygen and carbon dioxide atmosphere, the second oxygen and carbon dioxide atmosphere, and the third oxygen and carbon dioxide atmosphere, may in another aspect, be preheated to a temperature substantially equal to the first regeneration temperature, the second regeneration temperature, and the third regeneration temperature, respectively.
In an alternative embodiment, a carbon dioxide by-product of the first regeneration zone may be used at least as a portion of the second oxygen and carbon dioxide atmosphere. Likewise, in another embodiment, the second oxygen and carbon dioxide atmosphere is substantially water-free. For example, in an alternative embodiment, the second oxygen and carbon dioxide atmosphere has a water content ranging from about 1 to about 10 mole percent.
A catalytic cracking process is provided in another aspect of the invention. The process includes (1) introducing a feedstock and a catalyst into a catalytic cracker, (2) cracking the feedstock into a cracked product and coke, the coke forming a deposit on a spent cracking catalyst, (3) regenerating the spent cracking catalyst obtained from the catalytic cracker as outlined above, and (4) recycling a regenerated catalyst to the catalytic cracker. In an alternative aspect, the feedstock is unseparated feedstock containing asphalt or pitch. However, in another aspect, the feedstock is a preheated feedstock.
In still another aspect of the present invention, the formation of synthesis gas is maximized by (1) utilizing a preheated oxygen and carbon dioxide atmosphere in any of the first, second, or third regeneration zones, (2) avoiding the use of cooling coils in any of the first, second or third regeneration zones, therefore oxidizing, in the first reaction zone, only enough coke and supplemental fuel to oxidize the hydrogen content of the coke and the supplemental fuel to reduce the water content in the second regeneration zone so as to avoid exceeding the operating conditions which result in destroying the cracking catalyst. Such conditions are a function of the second and third regeneration zone""s temperatures and flue gas water content.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.